Continous vaccum casting process



Aug. 29, 1961 c. w. HANKS ET AL 2,997,760

CONTINUOUS VACUUM CASTING PROCESS- Filed June l0, 1957 2 Sheets-Sheet 1(oA/Tawau; 50p/LY 0: Mir/u (awww/N6 Vamr/LE Marra? MA/Nm/Nia VaLAr/LfH547' P P Vidya/w Afr/,va Him' Maw WML/4 Aug' 29 1951 c. w. HANKS ETAL2,997,760

CONTINUOUS VACUUM CASTING PROCESS Filed June 1o, 1957 2 Sheets-Sheet 2WMMP/ United States vPatent O This invention relates to the melting andcasting of metals in vacuo to remove co-mixed or dissolved volatilematter, and in particular to an improved vacuum devolatilizing andcasting process operable on a substantially continuous basis forproducing castings of de' volatilized metal at lower cost than has beenpossible heretofore. The present patent application is acontinuation-,in-part of our copending application entitled VacuumMelting and Casting Apparatus, Serial No. 603,508, led August 13, 1956,now abandoned.

Metals that have been melted at atmospheric pressure invariably containdissolved gases and other volatile matter (such as sulphur) to somedegree, and metal in sponge form from chemical processing operationsfrequently contains admixed volatile matter such as chlorides and theirassociated water of hydration, as Well as dissolved hydrogen. As hereinused, the term volatile matter refers to all impurities in the metalthat are volatilized, or form volatile compounds with other substancespresent, or decompose into volatile constituents when separated from themetal at temperatures near the melting point of the metal. It is wellknown that elimination of such volatile matter from metals in order toproduce sound ingots, with no porosity and a low content of impuritiesresulting from the presence of such volatile substances, is a desirableobjective for many metal working applications.

Vacuum melting and casting accomplishes this elimination of volatilematter to a degree that depends largely upon the absolute pressurereached in the vacuum tank. Thus, an increased degree ofdevolatilization occurs in a molten metal as the pressure on the metalis reduced, until a practical limit is reached at absolute pressures inthe order of one-half micron of mercury or less. :At these pressuressubstantially complete devolatilization of the molten metal occursalmost immediately, and reactions such as carbon de-oxidation of copperand iron occur practically instantaneously at the surface of the moltenmaterial. Consequently, ingots and other castings of metal that has beenmelted and cast in a high vacuum are substantially non-porous andcontain very little volatile matter. As herein used, the term highvacuum, refers to absolute pressures smaller than about one micron ofmercury, at which substantially complete devolatilization of the moltenmetal occurs.

In the high vacuum required for substantially complete devolatilizationof a metal, the amount of volatile matter evolved during the melting ofany appreciable quantity of metal previously processed at atmosphericpressure forms such a large volume of gas that continuous maintenance ofthe high vacuum' With practical pumpV congurations has not beenfeasible. ventional way to cast metals under high vacuum conditions hasbeen to operate batchwise, with relatively high.

pressures (1000 microns of mercury absolute pressure, for example) atthe onset of melting. As the gas evolution proceeds and graduallydiminishes, while the vacuum pump operates continuously, the pressure inthe vacuum system gradually drops and finally, after the system has beenat a pressure less than one-half micron of mercury for several minutes,devolatilization of the molten metal is essentially complete and thecasting operation is performed.

Consequently, the con- 2,997,760 Patented Aug. 29, 1961 .ICC

Although the conventional batchwise method for casting metal underhigh-vacuum conditions produces castings of high quality, the process isrelatively slow and the cost of the castings is high. In particular,batchwise pumping with absolute pressures in the vacuum tank varying bya factor of 1000 or more prevents economical utilization of the vacuumpumps.

Various attempts have been made previously to increase the speed and toreduce the cost of vacuum casting operations. However, to the best ofapplicants knowledge, no vacuum casting process orl apparatus prior tothe present invention has solved all of the following problems:continuous or semi-continuous operation for relatively economical andlarge-scale production of devolatilized castings; elimination ofbatchwise pumping and accompanying vacuum pump inefficiencies; castingin sufciently high vacuum for substantially complete devolatilization4of the metal; production of sound castings with virtually no cavities orfissures; handling commercialgrade metal initially containingsubstantial quantities of volatile matter (e.g., more than about 0.1percent by weight); and providing reasonably eflicient means for heatingand melting the metal.

Accordingly, an object of this invention is to provide an improvedvacuum melting and casting process that solves the aforesaid problemsand achieves the aforesaid and other advantages; and in particular toprovide improved and practical means for producing highly devolatilizedcastings on a substantially continuous basis at lower cost than has beenpossible heretofore; to eliminate batchwisepumping cycles and tomaintain a continuous high vacuum for the casting operation; to makepossible the use of electronic heating techniques that can be used onlyin high vacuum; and to avoid electrical instability and arcing that haveheretofore hindered the use of such techniques in vacuum castingoperations.

Certain metals, such as titanium, are so active chemically when they aremolten that it is hard to provide any container for holding them whilein the molten state. Accordingly, another object of this invention is toprovide improved means for the vacuum melting and casting of chemicallyactive metals such as titanium.

Briey stated, yin accordance with certain of its aspects, this inventionmakes practicable a continuous, economical, large-scale process fordevolatilizing and casting metals in vacuo, consisting of severalcontemporaneous operations on a continual llow of material.

Molten metal, initially containing substantial amounts of volatilematter, is continually supplied in contact with a continuouslymaintained rough vacuum, so that the metal continually evolves a portionof its volatile matter into the rough vacuum to produce a continualsupply of partly devolatilized molten metal. As herein used, the termrough vacuum refers to absolute pressures between approximately tenmicrons of mercury and apprixomately one millimeter of mercury. Withinthis vrange of pressures, a metal previously processed at atmosphericpressure will, when molten, evolve a major portion of the volatilematter that it contained initially; and the volatile matter so evolvedwill occupy only a small fraction of the volume that the same volatilematter would occupy in the much higher vacuum required for completedevolatlization of the metal. Consequently, a conventional vacuum pumpdesigned for operation at rough-vacuum pressures can, with relative easeand good pumping eiciency, maintain the rough vacuum. contin- 1 uouslywhile removing a major portion of the evolved volatile matter from thevacuum system. In this stage of the process, an absolute pressuresubstantially lower than ten microns of mercury would prevent ecientpumping of the volatile matter out of the Vacuum system, not onlybecause of the greater diculty in maintaining a higher vacuum, but alsobecause of the increase in volume of a gas as its pressure is lowered.An absolute pressure substantially higher than one millimeter of mercurywould leave too much volatile matter in the metal for eicient operationof the next stage of the process.

The partly devolatilized molten metal is continually transferred intocontact with a continuously maintained high vacuum, at an absolutepressure smaller than one micron of mercury, so that the metalcontinually evolves a remaining portion of the volatile matter into thehigh vacuum to produce a continual supply of highly devolatilized moltenmetal. Because a major portion of the volatile matter is evolved intothe rough vacuum before the metal is transferred into contact with thehigh vacuum, continuous maintenance of the high vacuum, while removingthe volatile matter continually evolved into it, can be accomplishedreadily and economically with available high-vacuum pumps.

The highly devolatilized metal may be vacuum cast in the same highvacuum that was used in producing it. The castings so made are of highquality, are non-porous, and contain very little, if any, volatilematter.

The new process can be carried out most effectively Within a partitionedvacuum tank defining upper and lower vacuum chambers. A rough vacuum iscontinuously maintained in the upper vacuum chamber by an oil jet pumpor the like, and a high vacuum is continuously maintained in the lowervacuum chamber by an oil diffusion pump or other suitable high-vacuumpump. Since each pump operates under substantially constant conditions,and seldom operates outside the pressure range for which it is bestsuited, both pumps can be designed for exceptionally efcient andeconomical pumping. In cases where a substantial part of the volatilematter is condensable at ambient temperatures, condensers may be used toreduce the volume of matter that must be removed by the pumps.

The metal can advantageously be melted in a small crucible fittedi intoand forming a part of the partition separating the upper and lowervacuum chambers. The top of the crucible opens into the upper vacuumchamber, so that a rough vacuum is continuously maintained within thecrucible. Metal is continually fed into the crucible and heated to itsmelting temperature, to provide a continual supply of molten metal incontact with the rough vacuum. The molten metal continually evolvesvolatile matter into the rough vacuum to produce a continual supply ofpartly devolatilized metal. The partly devolatilized metal continuallyows through an aperture in the bottom of the crucible into the lowervacuum chamber, where devolatilization is completed by the continualevolution of volatile matter into the high vacuum. The crucible can beheated by any convenient means to heat the metal therein. Since the`bottom of the crucible may extend into the lower or highvacuum chamber,electron bombardment of the crucible can be used advantageously forheating purposes, without difficulties due to electrical instability andarcing that are encountered at higher pressures.

In the case of titanium and other chemically active metals the use orf-acrucible to contain the metal in the upper chamber is made practical bythe fact that the metal can flow out of the crucible almost as soon asit melts, so that molten metal is in contact with the crucible for avery short time, and by the fact that the size of the crucible can bevery small compared to the quantity ofmaterial handled over a period oftime, so that the use of expensive chemically-resistant crucibles iseconomically feasible.

The metal flowing out of the bottom of the crucible may be received byan open-topped mold suitably positioned in the lower or high-vacuumchamber. For example, anV annular, water-cooled mold may be used-to castcontinuous billets orrods, which may be continually with drawn from thebottom of the mold. 'I'his method of casting is particularly desirablein the case of cherncally active metals such as titanium, because themolten metal flowing out of the crucible can be caught and held in askull formed at the top of the solidified cast rod of the same metal.Thus, chemical action on the mold and consequent contamination of thecast metal are substatially avoided. With less active metals such assteel, the metal flowing out of the crucible may be caught and heldinitially in a ladle or other vessel, and subsequently transferred,under high vacuum, to molds of any desired type and shape.

To insure a sound casting, particularly in the casting ofcontinuous rodswith a water-cooled annular mold, additional heat, must be suppliedcontinually to a pool of molten metal held in the skull at the top ofthe cast rod. This presents a rpoblem because heat must be supplieddirectly to the pool of molten metal and not directly to thewater-cooled mold. Thus, resistance heating, induction heating, and thelike are not feasible. The problem is solved by using high-voltageelectron bombardment to heat the pool of molten metal. The use ofVelectron bombardment is made possible by the continuous mainteance of ahigh vacuum in the lower vacuum chamber. The high vacuum permits theexistence of a space-charge limited electronic current, withoutelectrical instability or arcing.

The invention may be better understood from the following illustrative`description and the accompanying drawings. The scope of the inventionis defined by the appended. claims.

In the drawings:

FIG. 1 is a simplified flow diagram illustrating certain aspects of thisinvention;

FIG. 2 is a largely schematic vertical section of apparatus that may beused in practicing this invention; and

FIG. 3 is a fragmentary vertical section of other apparatus that may beused in practicing the invention.

The simplified flow diagram illustrated in FIG. l of the drawings isself-explanatory. It will be noted that the flow diagram describes, ingeneral terms, a substantially continuous 'process for makingdevolatilized castings. There is. a continual ow of material wherebymolten metal is brought into contact, sequentially, with a continuouslyAmaintained rough vacuum and a continuously maintained high vacuum.Preferably, heat is applied to thehmetal inf both vacuum stages to keepthe metal in a molten state, andthe volatile matter evolved by the metalis continuously pumped out of both vacuums. Finally, the devolatilizedmetal is vacuum cast in a mold, from whichheat is extracted.

Toy facilitate a better understanding of the invention and certainfurther aspects thereof, it will now be described in conjunction withapparatus that may be used in practicing the invention. It should beunderstood that the invention is not limited to any specific form orconstruction of such apparatus, except to the extent that the form ofthe apparaus isv dictated by the requirements of process, and thatnumerous variations are possible in the form and structure of theapparatus employed.

Referring to FIG. 2 of the drawings, a vacuum system enclosure 1 and Iahorizontal partition 2 therein dene adjacent upper and-lower vacuumchambers. A rough vacuum, at an absolute pressure between ten microns ofmercury and one millimeter of mercury, is continuously maintained in theupper chamber by a vacuum pump 3. A high vacuum, at an absolute pressureless than one micron of mercury, is continuously maintained in the lowerchamber-by a vacuum pump 4. Each of the two vacuum pumps. isfdesignedforbest operation under its particular conditions. For example, pump 3may advantageously bean oil jet pump which operates best in the rangeofsuction pressures betweenabout ten and three hundredvm'icrons ofmercury, absolute.l yPump 4v may be an oil dilusionpump (commerciallyavailable at present in sizes up to forty-eight inches diameter)designed for high-vacuum operation to maintain an absolute pressure ofabout one-half micron of mercury, or

ess.

An open-topped Crucible 5 is supported by and extends downward throughpartition 2, as shown, so that the interior of the Crucible communicateswith and is a part of the upper or rough-vacuum chamber while the loweroutside portion of the Crucible extends into the' lower or high-'vacuumchamber. Thus, in effect, Crucible 5 is a part of the partition betweenthe two vacuum Chambers. Partition 2 and Crucible 5 together separateand form `a vacuum seal between the rough-vacuum chamber and the:high-Vacuum chamber. The bottom of the crucible contains a smallaperture 6 (e.g., a one-eighth inch diameter hole in a one and one-halfinch inside diameter Crucible) through which molten metal may flow fromthe rough-vacuum chamber into the high-vacuum chamber. A plug 7,operable to Close aperture 6 by movement of an operating rod 8 extendingout of the vacuum system through a vacuum seal 9, may be provided tocontrol the flow of metal out of the Crucible. During operation, plug 7is raised suiciently to permit a continual flow of molten metal throughaperture 6.

The Crucible may be made of any suitable refractoryv material that canwithstand high temperatures land is chemically resistant to the metalbeing melted. For example, in the vacuum melting and Casting of iron orsteel or the like, Crucible 5 may be made of graphite lined inside withsuitable refractory material. Various means may be employed for heatingand melting the metal within the Crucible, such as induction heating orelectrical resistance heating, but preferably the Crucible is heated byelectron bombardment. For this purpose the Crucible is made of anelectrically conductive material, or is coated with an electricallyconductive material, and is electrically connected to ground throughmetal parts of the vacuum system enclosure or by any other means.

A substantially annular electron-emitting cathode 10 extends aroundCrucible 5. The Cathode may, for example, be a loop of tungsten wirehaving its ends connected to and supported by a pair of leads 11 and 12that extend through insulators 13 and 14 through a side wall of vacuumenclosure 1, as shown. Leads 11 and 12 may be connected to the secondaryof a transformer 1S having a primary connected to any convenient sourceo-f alternating current for providing a current through loop 10 to heatthe same and to produce thermionic emission of electrons at the cathode.The cathode 10 is maintained at a negative potential relative toCrucible 5 by any sui-table means, such Ias an electric generato-r 16connected to a center tap on the secondary of transformer 1S, as shown.Consequently, the Crucible is the anode of a high-vacuum diode, andelectrons emitted by cathode 10 areaccelerated to high velocities andbombard Crucible 5 to heat the Crucible and to melt metal Contained inthe Crucible. Since the electron discharge between cathode 10 andCrucible 5 occurs in a region of continuous high vacuum, there islittle, if any, tendency for electrical instability due to arcingbetween the cathode and the Crucible. Thus, the current betwen thecathode and the Crucible is essentially electronic, and is space-Chargelimited as inl vacuum-tube diodes.

Metal may be supplied to Crucible 5 by any convenient means appropriateto the kind of metal employed. For example, where steel or iron is to bemelted and cast, highquality steel or iron bar stock 17 may be fedcontinually into the upper portion of the Vacuum system through aconventional Vacuum seal 18. The lower end of bar 17 extends into themolten metal `19 Contained within crucible 5, and there it melts to]maintain within the Crucible a continual supply of molten metalinitially Containing a substantial amount of volatile matter.

v Since molten metal 19 is exposed to a rough vacuum through the opentop of Crucible 5, and the upper surface of the molten meta-1 is incontact with the rough vacuum, l

a major portion of the volatile matter is evolved by the molten metalinto the rough vacuum. At the pressure maintained within this portion ofthe vacuum system, the volume of the released volatile matter is notexcessive for economical pumping to maintain the rough vacuum.

In the purification of steel in particular, a refractory Crucible isemployed, and hydrogen dissolved in the steel Combines with oxygen fromthe refractory Crucible, forming water vapor which is evolved into therough vacuum. Aside from pumping problems, it can be noted that` a highvacuum at this stage of the puritication would be of no greater worththan a rough vacuum because of continual recontamination of the steel byoxygen from the Crucible. ln the process according to the presentinvention, such oxygen is removed in the subsequent highvacuum stage.

A plurality of heat shields 20, 21 and 22 may be provided above crucible5, as shown. The uppermost heat shield 22 may -advantageously containpassages 22 through which water or other cooling iluid may beCirculated. The heat shields provide several advantageous results,including the following: they help to retain heat in the Crucible, andthus to reduce the amount of electron bombardment necessary to keep theCrucible at the desired temperature; they reduce undesirable heating ofother portions of the vacuum system; and they act as a condenser thatsolidities condensable portions of the volatile matter released from themolten metal, and thereby reduces the volume of gas that must he removedfrom the system by vacuum pump 3.

Other heat shields 23, 24, 25, 26 and 27 substantially surround thelower portion of Crucible 5, as shown. The outermost shield 27 mayadvantageously be provided with passages 27 through which water or othercooling fluid may be circulated. In addition to its conventionalheatshielding and condensing functions, the shield structure 23-27 alsoserves as an electron-focusing electrode, as is hereinafter explained.

Molten metal Vfrom Crucible 5 flows continually through aperture 6 intothe high-vacuum chamber of the vacuum system. Drops 28 of the moltenmetal fall into a pool of molten metal 29 Cont-ained within a skullformed at the top of a bar 30 or ingot of the solidified metal. Forforming the skull of solidified metal, an annular casting mold 31,having an open top end opening into the high-vacuum cham-ber anddisposed vertically below `aperture 6 for receiving the metal flowingout of Crucible 5, is provided for cooling the circumference of bar 30,so that the molten metal in pool 29 solidifies from the circumferenceinward, forming a concave depression or skull in the solidied metal asindicated by the broken line at the top of bar 30, and a pool of moltenmetal is retained at the top of rod 30 in a manner analogous to theretention of a pool of molten wax at the top of a candle. Mold A31 ismade of a material having a high thermal Conductivity, such as copper,and preferably is provided with passages 31 through which water or otherCooling lluid may be circulated. Mold 31 is held in place within thevacuum enclosure by any suitable means, such as supporting posts 32 and33. As metal solidies at the top of rod 30, the rod may be withdrawncontinually 'from the open bottom end of the mold and removed from the-vacuurn system through a conventional vacuum seal 34.

Thus metal may be melted, `devolatilized and cast in a substantiallycontinuous manner. As bar 17 is lowered, metal, initially containing asubstantial amount of volatile matter, is continually supplied toCrucible 5 where the metal is melted and continually evolves a portionof its volatile matter into the rough vacuum to maintain a continualsupply of partly devolatilized molten metal 19. The partly devolatilizedmolten metal continually llows or drips through aperture 6 into the pool29 of molten metal contained in the skull at the top of rod 30, andcontinually evolves a remaining portion orf its volatile matter into thehigh vacuum. Thus, a continual supply of highly devola- 7 tilized moltenmetal is provided at the top of mold 31. As the metal in pool 29solidies, rod 30 is continually withdrawn through seal 34 to provide acontinual supply of devolatilized cast metal.

As hereinbefore explained, a major portion of the volatile matter isreleased from the metal while it is in the molten state within crucible5, and condensable portions of the volatile matter so released solidifyon heat shields 20 through 22 while noncondensable portions are pumpedout of the system by vacuum pump 3. In other words, a major portion ofthe volatile matter is eticiently removed from the metal at therelatively high absolute pressures existing in the rough vacuum chamber.However, because of these high pressures, the metal flowing throughaperture 6 into the high-vacuum chamber may still contain an appreciablequantity of volatile matter.

In the high vacuum existing below crucible 5, the remaining volatilematter is almost immediately evolved by the molten metal. Condensableportions of this volatile matter may solidify on heat shields 23 through27, and the remainder is removed from the vacuum system by vacuum pump4. Since a high vacuum is maintained continuously within the lowervacuum chamber, the cast metal is substantially completelydevolatilized.

To insure the production of a sound casting, free of cavities andiissures, the metal at the top of pool 29 must be maintained at atemperature well above the melting point. For this reason, and forfurther insurance that the cast metal will have been completelydevolatilized, heating of the metal in the high-vacuum chamber isessential. Furthermore, the pool of molten metal must be heateddirectly, without directly heating the copper mold. Therefore, commonheating methods, such as resistance heating, induction heating, and thelike, are not feasible. The problem is solved by employing electronbombardment heating.

Electron bombardment of the molten metal may be used as a heating meanswithout encountering serious problems of electrical instability or arcformation because this heating occurs in a continuously-maintained highvacuum. Thus, the maintenance of a continuous high vacuum -in the lowervacuum chamber makes feasible the use of electron-bombardment heatingwhich solves the problem of directly heating the pool of molten metalwithout directly heating the mold.

The pool of molten metal 29 is maintained at ground potential throughmold 31 and metal portions of the vacuum system that form a closedelectrical circuit between the mold and ground, or by any other means.An electron-emitting cathode 35 preferably is of annular form, andadvantageously may be a loop of tungsten wire having its ends connectedto and supported by leads 36 and 37 which extend through insulators 138and 39 through a side wall of vacuum system enclosure .-1. Leads 36 and37 are connected to the secondary of a transformer 4t) having a primaryconnected to any suitable source of alternating current for supplyingelectric current through cathode 35 to heat the same and to producethermionic emission of electrons. Cathode 35 is maintained at a largenegative potential (in the order of several thousand volts) relative topool 29 by any suitable means, such as an electric generator 41connected to a center tap on the secondary of transformer 40.Consequently, pool 2'9 is the Kanode of a high-vacuum diode, andelectrons emitted by cathode 35 are accelerated to high velocities andbombard the pool of molten metal 29 from above for maintaining the metalat the upper surface of the pool at a suicienttemperature,.substantially above the melting temperature of the metal,for the casting of a sound ingot.

It is desirable that the electrons emitted by cathode 35 be focused onpool 29 so that substantially all of the electrons emitted will be usedto heat the molten metal. Substantial electron bombardment of heatshields 23 through 2.7 or mold 31 would heat these parts, and thereforeis quite undesirable. For focusing the electrons, the heat-shieldstructure 2'3 through 27 may be made substantially to surround cathode35, as shown, with a central aperture in the bottom of the heat-shieldstructure alined with pool 29. The heat shield structure is supportedyand insulated from ground by electrical insulators 42 and `43, and itis maintained at the same potential as cathode B5 or at a negativepotential with respect to cathode 35 by any suitable means, such zas agenerator 44 and a lead 45 that extends through an insulator 46 througha side wall of Vacuum enclosure 1. The negative potential of the heatshield structure repels the electrons emitted by cathode 35 yandeffectively focuses the electrons on the pool 29 of molten metal.

Alternatively, but less desinably, instead of providing separate voltagesupply means for maintaining heat shields 23 through 27 negative withrespect to cathode 35 the heat shields may simply be electricallyinsulated from ground and other parts of the apparatus. Electrons maythen flow to the heat shields until the accumulated charge provides asuiilciently negative potential to stop such ilow.

Focusing the electrons onto pool 29 is also aided by the vaporsemanating from the pool of molten metal, which provide a relativelylow-resistance region immediately above pool 29, iby providing positiveions that partly neutralize the electronic space charge in the terminalportion only of the electron llow. This lowresistance region helps todirect the electron iow toward pool 29 and away from mold 31 and otherparts of the apparatus. However, it should be kept in mind that a highvacuum, at an absolute pressure less than one micron of mercury, iscontinuously maintained in the lower vacuum chamber. Therefore, theelectric current between cathode 35 and pool 29 is essentially ahigh-voltage spacecharge limited electron flow, such as occurs in aLhard vacuum tube, and heat is produced by the high-velocity electronbombardment of pool 29. The ionic current is negligible, and'appreciable space-charge neurtalization occurs only in a small regionimmediately above pool 29. A glow discharge, such as would prevail atrough-vacuum pressures, would not Ibe suitable `for several reasons,including the fact that the heat released by -a glow discharge could notbe -adequately concentrated on pool 29, and the fact that much greatercurrents would be required for la given heating rate, as well as controland stability diiculties.

If the linitial content of volatile matter in the metal is exceptionallyhigh, `as in the case of ordinary commercial steel, it might he moreeconomical to perform the out-gassing of the metal in three stagesrather than two. This can be accomplished, for example, by providingstill another Vacuum chamber above the upper chamber shown in FIG. 2,and maintaining progressively higher degrees of vacuum in the threechambers. In this case the absolute pressure in a rst or uppermostchamber is preferably -in the order of several millimeters of mercury (alow vacuum), the absolute pressure in the intermediate vacuum chamber issomewhat less than one millimeter of mercury (a rough vacuum as hereindened), and the absolute pressure in the iinal or lowest vacuum chamberis less than one micron of mercury (a high vacuum). The metal -is firstmelted in the lowest-vacuum (highest-pressure) chamber for preliminaryout-gassing and then is continually transferred by any suitable means tocrucible 5 in the intermediate or roughavacuum chamber for furtherout-gassing. Final devolatilization and casting of the metal in thehigh-vacuum chamber proceeds as hereinbefore described.

However, rather than provide a three-stage vacuum devolatilizationprocess, it is usually more economical to ruse a two-stage arrangementas illustrated in FIG. 2, and to use for the bar stock 17 high-qualityalloy steel, which is usually carefully de-gassed as muchas is possibleduring atmospheric operations, so that the quantity of volatile materialreleased in the rough-vacuum portion of the sys- ,9 tem will not exceedthe pumping capabilities'of vvacuumV pump 3.

It should also be appreciated that shapes other than bars or ingots canbe cast with minor modifications of the apparatus illustrated. Forexample, in the Casting of metals that are reasonably inert chemically,such as steel, instead of Containing the pool of molten metal 29 withina skull at the top of a bar 30, pool 29 may be contained in a holdingCrucible or ladle from which it can be supplied in vacuo to Castingmolds of any desired size and shape. In this case, the metal can be keptmolten by heating the holding crucible, instead of by direct electronbombardment of the molten metal.

Various other means for feeding metal into Crucible may be employed inplace of the solid bar stock 17. For example, the metal may be meltedinitially outside of the vacuum system and transferred to the Cruciblein a molten state, as by melting the metal in another Crucible disposedabove the upper vacuum chamber and causing the molten metal continuallyto drip into the open top of Crucible 5, in a manner analogous to theflow of molten metal from Crucible 5 into mold 3\1. Even in this case,heat preferably is supplied to the metal within Crucible 5 to keep itmolten, either by heating the crucible, as hereinbefore explained, or byheating the metal directly, by electron bombardment or other means.Alternatively, lumps' of unmolten metal may be fed continually intoCrucible 5 by any appropriate feed mechanism.

Where the metal being processed has an intense chemical activity whenmolten, as in the Case of titanium, Contact between the'molten metal and:the Crucible should be kept tota minimum. Otherwise, the Crucible willrapidly be destroyed and the metal will be contaminated by chemicalaction between the molten metal and the Crucible. This problem is solvedby feeding lumps of unmolten metal into the crucible, continuallymelting the metal within the crucible,V and permitting the molten metalto flow out of the Crucible almost immediately as the metal melts.

FIG. 3 of the drawings is a fragmentary view showing how the apparatuscan be adapted for handling metals such as titanium thatare extremelyactive chemically when in the molten state. A small open-topped Crucible47 extends through'and is supported -by a horizontal partition 48 withina vacuum system. Suitable means, not shown in FIG. 3, are "provided forcontinuously maintaining a rough vacuum `within Crucible 47 and a highvacuum below Crucible 47, in the manner hereinbefore explained. Heatshields, heating means, and an annular Casting mold are also provided.For Clarity,vthese parts have been omittedv from FIG. 3, since they maybe substantially like the corresponding parts illustrated in FIG. 2.Suicient heat is supplied to Crucible 47, preferably by high-velocityelectron bombardment, to heat metal within the Crucible to the meltingtemperature of titanium. Y

A supply of lumps 49 of titanium sponge is held in a hopper 50 disposedwithin the vacuum system. Such spongeis' the usual form of titaniumproduced by chemical pur icati0n processes, and may initially Containabout three-tenths to six-tenths of one percent, by weight, of magnesiumor sodium chloride, and a small amount of water absorbed by the chlorideimpurities. Both the chlorides and the water are undesirable volatilematter. For the fabrication of titanium parts by subsequent metalworking operations, sound Cast ingots of devolatilized titanium arerequired. Such ingots may advantageously be made by a vacuumdevolatilization and casting process incorporating principles of thepresent invention.

Any appropriate type of feed mechanism 51 is employed to withdraw lumpsof titanium sponge from hopper S0 and feed such lumps continually intothe open top of Crucible 47, as is indicated in the drawing by the lump52 of titanium sponge that is falling from the end of the feed mechanisminto the Crucible. Thus, a small supply of unmelted titanium sponge ismaintained within Crucible 47. Hopper 50 may be refilled from time totime with l 0 additional titanium sponge fed into the vacuum systemthrough a Conventional air lock, not shown.

The heat supplied to Crucible 47 continually melts titanium spongewithin the Crucible. The melting titanium Continually evolves a majorportion of its volatile matterA into the rough vacuum, and provideswithin Crucible 47 a con tinual supply of partly devolatilized moltentitanium. The volatile matter so evolved is condensed on the heatshields and other relatively Cool parts of the system, or is pumped outof the vacuum system or both.

A small aperture S3 is provided in the bottom of crucible 47, as shown.For example, Crucible 47may have an inside diameter of one and one-halfinches, and aperture 5-3 may be a one-eighth inch diameter hole. Becausethe aperture is small, very little of the volatile matter evolved withinCrucible 47 passes through aperture 53 into the high Vacuum below theCrucible.

Promptly after melting, the partly devolatilized molten titanium lflowsout of Crucible 47 through aperture S3 into the highl vacuum below theCrucible. A drop of molten titanium falling from the Crucible isindicated in the drawing at 54. Because each bit of molten titaniumremains within Crucible 47 for only a very short time after the titaniummelts, Chemical action between the molten titanium and the Crucible iskept to a minimum. Furthermore, because of the continuous nature of themelting and Casting process made possible by this invention, Crucible 47can be quite small compared to the quantity of titanium handled over aperiod of time. Consequently, it is economical to use expensive Cruciblematerials, such as' Cerium sulphide, 'that are relatively resistant tochemical attack by molten titanium. Therefore, this invention makeslarge-scale Crucible melting of titanium and the likeeconomicallyfeasible for the rst time.

The partly devolatilized molten titanium continually flowing out ofCrucible 47 through aperture 53 continually evolves a remaining portionof this volatile matter into the' high vacuum that is continuouslymaintained below the Crucible. Thus, a continual supply of highlydevolatilized titanium is provided. If desired, this highlydevolatilized titanium can be vacuum cast into rods or ingots in anannular mold like the mold illustrated in FIG. 2. Alternatively, thedrops 54 of highly devolatilized Ititanium can be permitted to losesuicient heat by radiation to solidify while falling through a highvacuum chamber. In this way pellets pf highly `devolatilized titaniumcan be produced. The devolatilized pellets can be removed from` thevacuum system through a Conventional air lock.

' It should be understood that this invention in its broader aspects isnot limited to specic examples herein illustrated and described. Thefollowing Claims are intended to cover all Changes and modicationswithin the true spirit and scope of the invention.

What is claimed is:

1. A continuous process for making devolatilized metal Castings Withinvacuum `apparatus including ia Crucible and a vacuum Chamber, saidCrucible being provided with an aperture opening into said Vacuumchamber for withdrawing molten metal from the Crucible, comprising thefollowing contemporaneous steps: maintaining a parti-al vacuum withinsaid Crucible at an absolute pressure between ten microns and onemillimeter of mercury; continually feeding into said Crucible metalinitially containing a substantial amount of volatile matter;continually heating said metal within said Crucible to the meltingtemperature of said metal, so that said met-al continually evolves aportion of said volatile matter into said partial vacuum to producepartly devolatilized molten metal continually within said Crucible;maintaining within said vacuum chamber a high vacuum at an absolutepressure of less than one micron of mercury; continually transferringsaid partly devolatilized molten metal through said aperture into anopentopped mold disposed directly below said aperture and within saidVacuum Chamber; supplying additional heat to said metal within saidvacuum Chamber by direct electron bombardment of the upper surface ofmolten metal within said mold, so that said metal continually evolves aremainlng portion of said volatile matter into said higher vacuum toproduce more highly devolatilized molten metal con tinually within saidvacuum chamber; and vacuum casting said more highly devolatilized metalto produce devolatilized metal castings.

2. A continuous process for casting devolatilized metal rods withinvacuum apparatus including a Crucible, a vacuum chamber, and an annularmold having open top and bottom ends, said Crucible being provided withan aperture opening into said vacuum chamber for withdrawing moltenmetal from the Crucible, the top end of said mold opening into saidvacuum chamber and being disposed vertically below said aperture forreceiving the molten metal withdrawn from said Crucible, Comprising thefollowing contemporaneous steps: maintaining a rough vacuum within saidCrucible; providing within said Crucible a continual supply of moltenmetal initially Containing a substantial amount of volatile matter, sothat said metal continually evolves a portion of said volatile matterinto said rough vacuum to produce a continual supply of partlydevolatilized molten metal; maintaining a high vacuum within said vacuumchamber; causing said partly devolatilized molten metal to flowcontinually through said aperture into the open top end of said mold, sothat said metal continually evolves a remaining portion of said volatilematter into said high Vacuum to produce a continual supply of highlydevolatilized metal within said mold; continually bombarding from abovewith electrons the upper surface of molten metal within said mold tomaintain a molten pool of said metal at the top end of said mold;continually Cooling said mold to solidify said highly devolatilizedmetal within the mold into a rod having a top end with a concavedepression supporting said molten pool; and continually withdrawing saidrod through the bottom end of said mold.

3. A continuous process for devolatilizing an active metal withinapparatus including an upper vacuum Chamber, a lower vacuum chamber, anda Crucible having an open top for receiving unmolten metal and anaperture for discharging molten metal, said top opening into said uppervacuum Chamber and said aperture opening into said lower vacuum Chamber,comprising the following contemporaneous steps: maintaining a roughvacuum within said upper vacuum chamber at an absolute pressure betweenten microns and one millimeter of mercury; continually feeding into thetop of said Crucible -a rod of unmolten metal containing substantialamounts of volatile matter; continually heating said Crucible to meltsaid rod continually as it is fed into said Crucible and maintaining acontinual supply of said metal in a molten state within said Crucible,so that said metal in a molten state continually evolves a portion ofsaid volatile matter into said rough vacuum to produce a continualsupply of partly devolatilized molten metal; maintaining a high vacuumwithin said lower vacuum Chamber at an absolute pressure of less thanone micron of mercury; causing said partly devolatilized molten metal toow continually through said aperture into an open-topped continuouscasting mold disposed directly below said aperture and within said lowervacuum chamber; and continually supplying additional heat to said metalwithin said lower vacuum charnber by direct electron bombardment of theupper surface of molten metal within said mold, so that said metalContinually evolves a remaining portion of said volatile matter intosaid high vacuum to produce a continual supply of highly devolatilizedmetal.

4. A continuous process for devolatilizing an active metal withinapparatus including an upper vacuum Chamber, a lower vacuum chamber, aCrucible having an open top and having a bottom containing an aperture,said top opening into said upper vacuum chamber and said apertureopening into said lower vacuum Chamber, comprising the followingcontemporaneous steps: maintaining a rough vacuum within said uppervacuum chamber and said Crucible at an absolute pressure between tenmicrons and one millimeter of mercury; continually feeding into the topof said Crucible lumps of unmolten metal Containing substantial amountsof volatile matter; continually heating said Crucible to melt said metalcontinually within said Crucible, so that said metal continually evolvesa portion of said volatile matter into said rough vacuum to produce acontinual supply of devolatilized molten metal within said Crucible;Causing said devolatilized molten metal to ow continually from saidCrucible through said aperture into an open-topped mold disposeddirectly below said aperture and Within said lower vacuum chamberpromptly upon the melting of said metal; maintaining a high vacuumwithin said lower vacuum Chamber; at an absolute pressure of less thanone micron of mercury and continually supplying additional heat to saidmetal within said lower vacuum Chamber by direct electron bombardment ofthe upper surface of molten metal within said mold, so that said metalcontinually evolves a remaining portion of said volatile matter intosaid high vacuum to produce a continual supply of highly devolatilizedmetal.

References Cited in the le of this patent UNITED STATES PATENTS2,423,729 Ruhle July 8, 1947 2,554,902 Godley May 29, 1951 2,625,719Moore Jan. 20, 1953 2,688,1469 Gruber Sept. 7, 1954 2,709,842 FindlayJune 7, 1955 2,771,568 Steigerwald Nov. 20, 1956 2,778,926 Schneiderlan. 22, 1957 2,804,664 Brennan Sept. 3, 1957 2,858,199 Larson Oct. 28,1958 2,880,483 Hanks et al. Apr. 7, 1959 2,882,570 Brennan Apr. 21, 1959FOREIGN PATENTS 413,743 Great Britain July 19, 1934 735,642 GreatBritain Aug. 24, 1955

1. A CONTINUOUS PROCESS FOR MAKING DEVOLATILIZED METAL CASTINGS WITHINVACUUM APPARATUS INCLUDING A CRUCIBLE AND A VACUUM CHAMBER, SAIDCRUCIBLE BEING PROVIDED WITH AN APERTURE OPENING INTO SAID VACUUMCHAMBER FOR WITHDRAWING MOLTEN METAL FROM THE CRUCIBLE, COMPRISING THEFOLLOWING CONTEMPORANEOUS STEPS: MAINTAINING A PARTIAL VACUUM WITHINSAID CRUCIBLE AT AN ABSOLUTE PRESSURE BETWEEN TEN MICRONS AND ONEMILLIMETER OF MERCURY, CONTINUALLY FEEDING INTO SAID CRUCIBLE METALINITIALLY CONTAINING A SUBSTANTIAL AMOUNT OF VOLATILE MATTER,CONTINUALLY HEATING SAID METAL WITHIN SAID CRUCIBLE TO THE MELTINGTEMPERATURE OF SAID METAL, SO THAT SAID METAL CONTINUALLY EVOLVES APORTION OF SAID VOLATILE MATTER INTO SAID PARTIAL VACUUM TO PRODUCEPARTLY DEVOLATILIZED MOLTEN METAL CONTINUALLY WITHIN SAID CRUCIBLE,MAINTAINING WITHIN SAID VACUUM CHAMBER A HIGH VACUUM AT AN ABSOLUTEPRESSURE OF LESS THAN ONE MICRON OF MERCURY, CONTINUALLY TRANSFERRINGSAID PARTLY DEVOLATILIZED MOLTEN METAL THROUGH SAID APERTURE INTO ANOPEN-TOPPED MOLD DISPOSED DIRECTLY BELOW SAID APERTURE AND WITHIN SAIDVACUUM CHAMBER, SUPPLYING ADDITIONAL HEAT TO SAID METAL WITHIN SAIDVACUUM CHAMBER BY DIRECT ELECTRON BOMBARDMENT OF THE UPPER SURFACE OFMOLTEN METAL WITHIN SAID